In Twitter's SEC disclosures, prior to its IPO this week, the company said it has 232 million users. But those aren't its total number of registered users. Instead, they are what Twitter calls "monthly active users."

Twitter has never disclosed its total number of registered users.

Traditionally, social media companies have focused on active users rather than total users because active users are the more meaningful measurement of how big or popular a service is. Twitter's monthly active user statistics show who is actually using the service, and create its value for other users and advertisers. It's the most important metric.

It has been known for a while that this dark pool of non-users was an issue. Wall Street analysts asked the question but got no answers on Twitter's IPO roadshow. AdExchanger reported:

“You don’t know how many people sign up and don’t use it, how many abandoned accounts they have,” said Adam Grossman, an analyst at Middleton & Co who attended a roadshow lunch presentation by Twitter executives in Boston on Thursday.

A source familiar with Twitter's advertising business tells us that only 60% of Twitter's users are active. About 40% are inactive — meaning that they simply "listen" to other tweets but never tweet themselves.

Those users — the people who follow others but attract almost no followers themselves — are still counted as monthly active users of Twitter. Signing in to read others' tweets still counts as "active." But those users are less valuable to advertisers. They engage less, and advertisers pay Twitter for engagement, not mere exposure.

And those inactive users are not the same as the total number of registered users, which would include people who have abandoned Twitter completely.

The dark pool is not, per se, a problem for Twitter. The company's IPO disclosures are right to concentrate on the active users — the total user population that can actually be monetized. Plenty of social media networks have inactive members, of course (most famously LinkedIn, whose members tend to check the site only when they need jobs).